Allie, I would check out Costco. I like their warranty policy. TV you buy has one year warranty, Costco's Concierge Service adds another year. If you use your Costco AmEx card, you get a 3rd year warranty at no additional cost. The selection of TV's is not as great as Best Buy, but you can find what you need there. Also, Costco.com has a little more selection than the stores. Best Buy's extended warranties will run between $100-140 for 4 years. To me, the warranty in itself is enough to send me to Costco for my TV's. I'm not sure if Sam's Club works the same way. Maybe a Sam's Club member could provide some further information.
Next you need to decide if you want an LCD, an LED LCD, or a plasma. Although they are harder to find, I have a DLP which I absolutely love, however, the DLP's are not nearly as thin as LCD's, LED-LCD's, or plasmas, but I find the picture to be better. Does your computer have an HDMI output? I believe most newer TV's will accommodate a computer, even if it does not have an HDMI output. I like to look at the customer reviews at the website I am researching what I buy, Amazon, Costco, etc. Consumer Reports is also good for their ratings and it is the bible of product reviews as well and they have their product ratings on their website (Samsung is their top 40-42" LCD). If you watch more high action stuff (movies, sports, etc), you may want to look at a TV with a higher Hertz (Hz) rating. Standard are 60 Hz, but some TVs have 120 Hz a few have 240 Hz. I have seen 480 Hz, but you probably don't need that, nor would you want to pay the premium it probably costs.
Swarmy is right about plasmas. The larger the screen, the more you would probably want a plasma over an LCD/LED-LCD. However, I would still take a DLP over a plasma for a big screen.
Here are a few at Costco you might want to check out.
Samsung 40" LCD
Vizio 42" LED LCD
Vizio 42" LCD
Panasonci 42"
Allie, I hope this helps you. There are a lot of other factors to think of as well. Is the room where you will be putting the TV well lit (lots of windows)? Plasmas tend to have a screen that will reflect more of a glare in higher light and you may find this annoying. LCD's tend to have less glare in higher light. Are you going to mount the TV on the wall or on a stand? PM me with any other questions.